This invention relates to the designing of a conical diffracting oblique incidence spectroscope and a diffraction grating with non-uniformly spaced grooves that is suitable for the spectroscope.
In a conventional spectroscopic system using a diffraction grating to perform wavelength scan, the diffraction grating is rotated about an axis that is parallel to the grating grooves and which is perpendicular to the plane formed by the incident and the diffracted principal rays. A problem with this system is that as the angle of rotation increases, the shadowing effect, the phenomenon that a part of incident or diffracted beam is blocked by the grating facet, increases and the diffraction efficiency decreases with increasing wavelength. In order to overcome this difficulty, it has been proposed that wavelength scan be performed by rotating the diffraction grating about an axis normal to the grating surface (see, for example, U.S. Pat. No. 5,274,435 to M. C. Hettrick under the title of “Grating monochromators and spectrometers based on surface normal rotation, Monochromator with concave grating”). This method achieves some improvement in diffraction efficiency but, on the other hand, the wavelength scanning range is narrow and the resolving power decreases with increasing wavelength.
In another type of the conventional spectroscopic system which consists basically of a single diffraction grating, an entrance slit and an exit slit, and in which the plane formed by the centers of the entrance slit, the exit slit and the diffraction grating is not perpendicular to the direction of grooves in the diffraction grating. In this arrangement called “off-plane” mounting, substantial aberrations appear as long as the diffraction grating has uniformly spaced parallel grooves. As an improvement, it has been proposed to use a holographic concave diffraction grating having non-uniformly spaced, non-parallel grooves (U.S. Pat. No. 4,455,088 to M. Koike under the title of “Monochromator with concave grating”). However, even in this method, correction of aberrations is very difficult to achieve if it adopts the extreme “off-plane” mounting which is required by the grazing incidence spectroscope.